This project tests five hypotheses on the relative risks of HIV transmission due to syringe and ancillary paraphernalia sharing within a network of homeless heroin injectors in San Francisco. It seeks to explain the internal logics and structural causes for sharing by documenting the intimate mechanisms and social construction of risky injection practices in a fully indigenous street setting. Special attention is paid to the economic, biological and emotional imperatives faced by chronic heroin users. Data will be collected by a multi-ethnic research team utilizing intensive, around-the-clock participant-observation methods conducted inside the shooting galleries and sleeping encampments of a core network of twelve heroin abusers and their peripheral social contacts (approximately 50 additional drug users street hustlers, and day laborers). The bulk of the ethnographic data will consist of direct observations and tape-recordings, but will also include photographs, videotapes, and HIV tests. Understanding and explaining the logics for injection practices will enable the delineation of profiles of relative risk for HIV infection both for individuals and for types of social networks. Through HIV seroprevalence testing of individuals and paraphernalia, the project will identify which sharing practices are riskiest and why certain types of individuals and/or social networks are at greatest risk. Rather than making statistical correlations, the project aims to understand and explain risky injection behaviors. An analysis of the pragmatics and contexts for dangerous behaviors that are often concealed or misreported to outsiders will offer public health service providers an indigenous perspective on the effectiveness of their outreach programs. By complementing San Francisco's extensive epidemiological documentation with participant-observation data and analysis, the project hopes to promote a methodological dialogue capable of augmenting the accuracy of future data collection by improving survey protocols, reframing questions and hypotheses, and systematizing sampling strategies. The long-term goal is to contribute to an explanatory model for variance in HIV infection rates in different vulnerable sectors of the U.S. population.